1. Field of the Invention
The present invention relates to a method of recovering L-threonine from the fermentation broth of an L-threonine producing microorganism by using a nonsolvent, and more particularly, to a method of recovering crystalline L-threonine by using a drowning-out process in which the fermentation broth of an L-threonine producing microorganism is filtered and the filtate is concentrated and reacted with a nonsolvent to crystallize L-threonine, crystalline L-threonine recovered using the method, and a feed additive containing the crystalline L-threonine.
2. Description of the Related Art
L-threonine, which is an essential amino acid, is mainly used in complete amino acid formulations, nutritional supplements, and the like. Recently, L-threonine has been used as an additive to animal feed together with L-lysine, and thus the demand for L-threonine has increased.
L-threonine is mainly produced by fermentation and concentration-crystallization. That is, microbial bodies are removed from the fermentation broth of L-threonine producing microorganisms by filtration or centrifugal separation, the pH of the obtained filtrate is adjusted, and then the pH-adjusted filtrate is concentrated with solvent being removed to recover crystalline L-threonine in the form of a needle.
However, when the concentration-crystallization method is used, the recovery yield of crystalline L-threonine is low. In the concentration-crystallization method, to maintain separation efficiency of concentrated crystalline slurry and product quality, an L-threonine containing solution or broth is concentrated to the range of 50 to 70% of L-threonine. Since crystallization is performed by heating the filtrate of an L-threonine containing fermentation broth to evaporate a solvent, crystallization proceeds at a high temperature, and a large amount of L-threonine is dissolved in the filtrate, i.e, the mother solution accordingly. Thus, when the crystalline slurry is separated using a separator, a relatively large amount of L-threonine remains in the mother solution, and thus its recovery is impossible.
In addition, the needle-form of the crystalline L-threonine, generated in the concentration-crystallization method, creates a big disadvantage. In the absence of additives or nonsolvents, the crystalline form of L-threonine is thin, long, and needle shaped. Therefore, the crystalline L-threonine formed in the concentration-crystallization method is in the form of thin needle with a length of about 50 to 250 μm. Agglomeration in the crystalline slurry occurs between the L-threonine crystals in the form of a needle, and thereby the viscosity of the crystalline slurry is increased, resulting in a reduction in separation efficiency when the crystals are separated. If the separation efficiency is decreased, L-threonine productivity is decreased and impurities remain in L-threonine products, and thus the product quality is degraded. The L-threonine crystals in the form of a needle also reduce the flowability of the product, and thus this causes a lot of inconveniences when consumers use the product. Generally, consumers use L-threonine as an additive to animal feed. L-threonine is added to animal feed using a manual or automatic system. Here, a reduction in the flowability of L-threonine causes problems in use of such a manual or automatic animal feed mixing system, and this often leads to claims of consumers. In addition, the strong tendency of the L-threonine crystals in the form of a needle to agglomerate leads to lumping and caking during long-term storage and transportation. Due to the agglomeration, consumers may sometimes have to process the agglomerated L-threonine products to obtain L-threonine before using them. In an era of global competition, any claims from consumers and consequent harm to product reputation may constitute as serious problems as weaken the foundation of business. Thus, the low flowability of the L-threonine crystals in the form of a pillar can serve as a serious disadvantage.
Korean Patent Publication No. 2000-0013855 discloses a method of purifying L-threonine from an L-threonine fermentation broth by recrystallization. However, this method is performed in four steps so that the process is excessively complicated with a lot of equipment required. In addition, an ion exchange resin tower is used in order to recover the loss of a mother solution, and thus a large amount of acid, base and water is used. In addition, in all steps of crystallization, a concentration-crystallization process is used to form the L-threonine crystals in the form of a needle, resulting in low flowability of L-threonine products.
Korean Patent Publication No. 2000-0013854 discloses a method of purifying L-threonine using electrodialysis. This method may reduce an amount of acid, base and water used compared to a conventional ion exchange resin method. However, since an L-threonine solution passing through an electrodialyzer is concentrated under reduced pressure to produce an L-threonine product in the form of a needle, the flowability of the product is low. In addition, compared to the case of directly concentrating a filtrate of a microbial fermentation broth, it takes over 6 hours to pass through the electrodialyzer. Thus, the productivity of the L-threonine product is low.
Therefore, there is still a need to develop an improved method of recovering L-threonine, which can solve the inconveniences of consumers that are caused by the low recovery yield of L-threonine from fermentation broth, and the low flowability of the L-threonine product as a result of the L-threonine crystal shapes.